K9 suddenly wants to encrypt all mail, and won’t send any

Obviously, I have inadvertently changed some setting, but can’t figure out what.

I use K9 with 6 accounts, none of which have previously used encryption (I would prefer to – my correspondents wouldn’t). Suddenly all of them, when I try to compose an email have a red padlock at the top right and a tooltip appears at the bottom saying

“Cannot connect to crypto provider; check your settings or click crypto icon to retry”.

On the individual account settings, and also on global settings, the Cryptography tab shows :

  • OpenPGP App – None
  • Show unencrypted signatures – unchecked
  • Autocryptography Mutual Mode – unchecked.

I cannot find any other cryptography options and do not remember changing any. What might have caused it (a very slim chance, but I can think of nothing else) is that it first occurred when I used FireFox Focus, which is privacy oriented and always runs in private mode, to share a link to a web page. Perhaps that changed some setting?

Well, it doesn’t really matter how it happened, but now I cannot send any mail without this error message.

How can I disable cryptography in K9?